In recent years, the application fields of the FRP structure are expanding in the industrial products. As the merits of the FRP structure, a point of reducing the number of parts and the number of assemble steps and a point of lightening of a structure can be given.
In the FRP structure, a request for the reliability to strength is high, too. Therefore, it is important to carry out a non-destructive inspection to the FRP structure after shaping and to correctly measure the existence or non-existence of any defect. When the non-destructive inspection is carried out, a standard test piece having an artificial defect formed is necessary, and especially, it is desirable to use a standard test piece having the defect like a defect which is possibly generated. One of the defect examples which occur in an FRP structure is demolding, and the most excellent artificial defect of the standard test piece is a space (regardless of existence or non-existence of gas). The space of this artificial defect blocks off supersonic communication in the ultrasonic test in case of a transmission method, and efficiently reflects the supersonic wave in case of a reflection method.
Conventionally, as a structure of the artificial defect material and a manufacturing method, Patent Literature 1 (JP H10-227773A) describes the artificial defect material in which glass minute hollow spheres having the spherical diameter of 30-150 μm are uniformly distributed in a combination base material made of silicone rubber having the thickness 0.1-0.3 mm.
The glass minute hollow sphere in Patent Literature 1 (JP H10-227773A) has a gas therein. Because the silicone rubber which covers the glass sphere has a comparatively good supersonic characteristic, the excellent supersonic characteristics can be obtained in the supersonic test of the transmission method and the reflecting method.
In para. [0006] of Patent Literature 1 (JP H10-227773A), it is described to burry a polytetrafluoroethylene film which has the acoustic impedance different from the material, in the artificial defect material.
Also, when the FRP structure is conventionally to be shaped as a unitary body, the positions where the space should be formed are present often between the layers of fiber reinforcing base material like a corrugate structure. When the FRP structure having an interlayer space should be shaped, there are problems of how to apply a pressure in shaping and a combination of a jig and a core.
Conventionally, as a method of forming a space between the layers of the FRP structure, Patent Literature 2 (JP H08-52812A) discloses a method of manufacturing a composite material shaping body in which a composite material prepreg is arranged in an outer mold, a mandrel jig sealed with an outer skin material and filled with silicon oil (a high expansion material) is installed into the interlayer space, and silicon oil expands through heating. In this manufacturing method, by the expansion of silicon oil, the composite material prepreg is pressure shaped from it inside.